Magnetic Separation and Recycling of Goethite and Calcium Sulfate in

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Magnetic separation and recycling of goethite and calcium sulfate in zinc hydrometallurgy in the presence of maghemite fine particles Tong Yue, Zhenghe Xu, Yuehua Hu, Haisheng Han, and Wei Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03856 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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ACS Sustainable Chemistry & Engineering

Magnetic separation and recycling of goethite and calcium sulfate in zinc hydrometallurgy in the presence of maghemite fine particles Tong Yue1,2, Zhenghe Xu2, Yuehua Hu1, Haisheng Han1, Wei Sun1* 1: School of Minerals Processing and Bioengineering, Central South University, Changsha, China 410083 2: Department of Chemical and Material Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 1H9

*Corresponding author: E-mail address: [email protected] (W. Sun)

1 ACS Paragon Plus Environment

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ABSTRACT Goethite (α-FeOOH) and calcium sulfate (CaSO4 2H2O) are the main solid constituents of the iron oxide residues in zinc hydrometallurgy by the goethite process. Impounding these residues in tailings ponds is costly to maintain and causes a great threat to the local environment. In this study, magnetic separation was applied to separating goethite from calcium sulfate with maghemite (γ-Fe2O3) fine particles as the carrier, which were prepared by roasting -1 µm pure magnetite mineral particles. The SEM images and XRD patterns indicated the precipitation of goethite on maghemite fine particles in the goethite process, which made the goethite aggregates magnetic, while the calcium sulfate formed nonmagnetic bulk precipitates. The magnetic goethite-maghemite aggregates were then separated effectively from calcium sulfate precipitates using a magnetic drum separator. The recovery of Fe and Ca to their corresponding products was 93.2% and 91.9%, respectively. The removal of S and As from goethite precipitates was studied by roasting with coal powder. Under the optimum conditions of the coke powder to the residue mass ratio of 4% and 1100 °C, 99.3% S and 99.5% As were removed while the goethite precipitates reached the standard of ironmaking raw materials. After drying, the calcium sulfate precipitates are used to produce cement and building materials. Keywords: Iron removal; Residue recycle; Surface precipitation; Magnetic particle; Arsenic and sulfur removal Introduction Iron is present as an undesirable constituent of zinc concentrates, calcine oxides and zinc oxide dust. In the leaching process of concentrates, calcines or dust, iron dissolves into the solution with zinc and other desired metals. Iron constitutes a severe impurity in zinc solution and must be removed before zinc electrolysis.1 In most existing electrolytic zinc plants, iron is removed by its precipitation as jarosite2, 3 and goethite4. Impounding a large amount of residues from the jarosite process in tailings ponds incurs high operating cost. The exposure of these residues to atmospheric conditions will cause severe environmental problems, due to the presence of heavy metals and/or hazardous elements, such as Zn, Ge, In, Pb, Se and S.5,

6

Furthermore, it is difficult to use jarosite residue because the Fe content in jarosite is lower than 30% with high fluctuation in the content of other undesired elements.1 Compared with jarosite residue, goethite residue has a lower volume and a higher Fe content (more than 40%), and


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contain a small amount of impurity elements.7 At the same time, it is possible to use this residue as an iron source in iron making after suitable treatments.8 Most electrolytic zinc plants use calcium hydroxide or calcium carbonate as neutralizer in the goethite process, which introduces no impurity into the solution and has a relatively low cost. However, using calcium carbonate or calcium hydroxide led to co-precipitation of calcium sulfate with goethite precipitates, resulting in the difficulty in utilizing these residues. As a result, the residues have to be impounded in tailings ponds which pose great threat to local soil, water and atmosphere by contained heavy metals and/or hazardous elements in the residue, such as Zn, Ge, As and S. To treat these progressively expanding hazardous residues, a novel method is developed in this study to convert these hazardous residues into secondary resources. Magnetic separation as a highly selective and extremely efficient technique to separate magnetic particles from solution or nonmagnetic particles9-13 and widely used in various areas, such as water treatment, biotechnology and ore refinement14-22 is considered in this method. To accomplish this objective, maghemite fine particles are introduced first into the goethite process as the nucleus seeds to promote goethite precipitation, imposing the required magnetism of goethite precipitates from the attached maghemite fine particles. The magnetically responsive goethite precipitatemaghemite composite particles are separated from calcium sulfate precipitates using a magnetic drum separator. After appropriate treatment to remove the hazardous impurities, the produced goethite-maghemite composite particles and calcium sulfate by magnetic separation can be used as ironmaking raw material and building material, respectively. This paper reports the results of addressing a long-term impoundment problem of the hazardous iron residues in smelters with our novel method. Experimental methods Materials and reagents A sulfuric acid leaching solution of zinc oxide dust obtained from one of the electric zinc smelters of the Yunnan Chihong Zinc & Germanium Co Ltd in China was used for iron removal by the goethite process. The leach residue of zinc calcine was placed in a fuming furnace to produce zinc oxide dust the reduction smelting. The zinc oxide produced as such was then leached in 160 g/L sulfuric acid solution under stirring at 500 rpm for 90 min. The initial solid dust content was kept at 125 g/L and leaching temperature was maintained at 80 °C. The leachate



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of zinc oxide dust with the pH in the range of 2.0 to 3.0 was obtained after the solid-liquid separation. The chemical composition of this solution shown in Table 1 was determined by the Testing Center of the Co Ltd using spectrophotometry with specific coupling agents for specific ions. It is worth mentioning that since a large amount of coal fines were blown in the furnace as the fuel and reductant in the smelting process of the leach residue of zinc calcine in the fuming furnace, the iron in the produced zinc oxide dust was mainly in the form of FeO. Since the dissolved oxygen content in the leaching solution was less than 5 mg/L at 80 °C, most of the iron in the leachate of this zinc oxide dust was mainly in the form of ferrous ions. Table 1. Chemical composition in the leachate of zinc oxide dust for iron removal. Chemical components

Fe

Zn

Ge

As

SO42-

TOC

Concentration mg/L

2496

125000

1.65

472

185000

135.4

Maghemite fine particles were produced from a high purity natural magnetite ore. The magnetite ore was first milled to less than 1 mm by a roll crusher. The milled ore was then ground by a wet stirred ball mill at 70% solids content until the particle size less than 1 µm. The Fe3O4 fine particles obtained as such were dried in a vacuum oven at 50 °C. Finally, the fine particles were roasted at 250 °C for 4 h in an adequate air atmosphere, during the color of the particles changed from black to red-brown, indicating the conversion of magnetite to maghemite. The FT-IR spectra of magnetite fine particles after thermal treatment at 50 °C and 250 °C were recorded between 400 cm-1 and 4000 cm-1 and shown in Fig. 1. The broad IR band at around 3430 cm-1 is assigned to the stretching modes of hydrogen-bonded H2O molecules or – OH groups on the surface of the fine particles and the IR band at around 1627 cm−1 is attributed to the bending vibrations of H2O.23, 24 The band at 565.5 cm-1 is reported as characteristic of the Fe-O bond of bulk Fe3O4.24, 25 After calcination at 250 °C for 4h, the band at 565.5 cm-1 splits into two peaks at 635.9 cm-1 and 556.1 cm-1, which are characteristic of maghemite (γ-Fe2O3).26, 27

The IR results confirm transformation of -1 µm Fe3O4 fine particles into γ-Fe2O3 after roast at

250 °C for 4 h in the adequate air atmosphere as a result of the following oxidation reaction. (1)


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Iron removal from zinc oxide dust leachate by the goethite process Maghemite fine particles were used in the laboratory test under the industrial production conditions of iron removal by the goethite process. In brief, 1 L of sulfuric acid leachate of zinc oxide dust was introduced into a 2-L jacketed pilot plant reactor and heated to keep at 85

by

4

water bath. High temperature is known to benefits the formation of goethite precipitates. Added into the leachate were 1-7 g/L of maghemite fine particles, with the mixture being kept under stirring by a mechanical mixer at 200 rpm. Substituting for oxygen-enriched air environment in industry, 10 mL H2O2 solution (6 wt.%) as the oxidant (representing a 22.9% excess) was pumped into the vessel and reacted for 60 min to oxidize all the ferrous ions to ferric ions in solution as shown in reaction (2). The fraction of hydrogen peroxide that contributed to oxidation was 67.1%. The pH during the reaction was maintained between 4.0-4.5 by pumping 50 g/L Ca(OH)2 slurry into the system. In the goethite process, a large amount of calcium sulfate dihydrate (CaSO4 2H2O, gypsum) and goethite (α-FeOOH) precipitates were generated as shown by reactions (3) and (4), respectively. (2) (3) (4) 50oC

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1627.53 3430.17 250oC 1627.93 565.52

3426.28

635.90 556.09

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 1. FTIR spectra of the magnetite fine particles after roasting at 50 °C and 250 °C. 5 ACS Paragon Plus Environment


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Magnetic separation and reduction roasting After crystallization by the goethite process, the suspension was pumped into a 30×30 cm magnetic drum separator to separate the magnetic particles from the suspension. The flow of the suspension was 100 ml/min. The magnetic drum separator uses electromagnets to generate magnetic field and its magnetic field strength can be adjusted from 0 Gs to 3000 Gs. After the magnetic separation, the magnetic concentrate and nonmagnetic tailings were obtained after solid-liquid separation by filtration. Both products were soaked in pH 3.0 sulfuric acid solution for 30 min under vigorous stirring at the solid-liquid mass ratio of 1:5 to remove the adsorbed zinc ions from particles surfaces. It should be noted that the zinc co-precipitated inside goethite and gypsum cannot be removed by this acid washing. After filtration and drying at 80

for 12

h goethite (the concentrate) and gypsum (the tailings) particles were obtained that could be potentially used as raw materials for value-added products.

Fig. 2. Schematic illustration of magnetic separation and production of desired goethite and gypsum product. Finally, reduction roasting was used to remove the impurity elements such as S and As from goethite. In this case, goethite was mixed with 1-6 wt.% of coke powder to pellet into 6 mm – 8 mm balls with the water as the binder by capillary force. An alundum crucible filled with these 6 ACS Paragon Plus Environment

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balls was roasted at 900-1150

for 30 min under N2 gas flow through the sealed roaster at 10

L/min flow rate. The tail gas was cooled by circulating water and collected by 1 mol/L sodium hydroxide solution. Figure 2 is a schematic illustration of the entire process. Results and discussion Magnetic separation of goethite and gypsum formed in our novel goethite process SEM images and XRD patterns of the concentrate and tailing of magnetic separation are shown in Fig. 3. As can be seen in Fig. 3 (c), the main components of the concentrate are maghemite, goethite and gypsum in the form of spherical aggregations as shown in Fig. 3 (a). The aggregates are formed by many goethite particles precipitating on the surface of maghemite as shown in the inset of Fig. 3 (a). Iron removal by the goethite process is through the formation of goethite crystals from ferric ions. In this process, the ferrous ions in the leaching solution were gradually oxidized to ferric cations by the oxidant, while the pH of the solution was maintained in the range of 4.0-4.5 to ensure that the ferric ions generated were in the state of supersaturation and formed rapidly the goethite precipitates.4 Maghemite fine particles in this process serve as the seeds for heterogeneous nucleation and crystallization. In the presence of fine maghemite particles, both homogeneous nucleation (nucleation in solution) and heterogeneous nucleation (nucleation on maghemite surface) could lead to formation of goethite. According to nucleation theory28, the abundant active sites on maghemite favor heterogeneous nucleation of goethite over its homogeneous nucleation in bulk, leading to the formation of goethite precipitates on maghemite as desired. As a result, the composite particles of maghemite and goethite precipitates are magnetic and can be separated effectively by magnetic separation. However, the existence of tannin (organic anion used for germanium precipitation) and high concentration of sulfate anion in the solution make it difficult to form the goethite of high crystallinity. As a result, the diffraction peaks of goethite are relatively low as show in Fig. 3 (c) while goethite precipitates on the maghemite surface in SEM image (Fig. 3 (a)) show no sharp angular outline. In contrast, most of the tailings precipitate from magnetic separation is pure needle-shaped gypsum, as shown in the SEM image (Fig. 3 (b)) and XRD pattern (Fig. 3 (d)). There is only a small amount of ultrafine goethite particles precipitated on the gypsum needle surface.



300

c

☆

△

☆: Maghemite (Fe2O3)

3500

△: Gypsum (CaSO4·2(H2O)) □□□□ : Goethite (FeOOH)

3000

Intensity (Counts)

350

Intensity (Counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

250 200

☆

150

☆ 100 50 0

☆

□ △

☆

△

△□

□

☆

☆

d

△: Gypsum (CaSO4·2(H2O))

2500 2000 1500 △

1000

☆

△

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△

△

500

☆

☆

△

△

△△

0 10

20

30

40

50

60

70

80

10

90

20

Two-Theta

30

40 50 Two-Theta

60

70

80

90

Fig. 3. SEM images of the concentrate (a) and the tailing (b) of the magnetic separation with 5 g/L maghemite and corresponding XRD patterns of the concentrate (c) and the tailing (d). The effect of maghemite addition on production of goethite and magnetic separation was studied. Fe and Ca content in the concentrates and tailings of magnetic separation in 2000 Gs magnetic field were analyzed by X-ray fluorescence (XRF). As shown in Table 2, without addition of maghemite particles, the precipitates generated in the goethite process are not magnetic and report to the tailings of magnetic separation. The tailings collected as such contain 14.96 wt.% Fe, 12.93 wt.% Ca, 15.43 wt.% S, 51.46 wt.% O and 2.74 wt.% As. This composition is very similar to the composition of goethite precipitates of electric zinc plant. As a result, they have to be impounded in tailings ponds. The mass of the concentrates increases with increasing the dosage of maghemite. Fe content in the concentrate is around 45% while the Ca content of the concentrate is kept around 3%. It is conceivable that adding more maghemite particles would provide more crystal nuclei to increase the ratio of heterogeneous nucleation to homogeneous nucleation, resulting in an increased mass of goethite precipitates on the maghemite. Since the amount of goethite precipitates that could be generated is fixed for a given iron content, the increased goethite content in the concentrates leads to an obvious decrease of Fe


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content and an increase of Ca content in the nonmagnetic precipitates reporting to the tailings. As anticipated, the benefit of adding more maghemite on inducing heterogeneous goethite precipitation reduces with increasing maghemite dosages, as shown by declining mass ratio of goethite to maghemite (G/M) in the concentrate from 1.78 at 1 g/L to 1.03 at 7 g/L dosage. Table 2. Fe and Ca content in the concentrates and tailings of magnetic separation with different dosages of maghemite in goethite process of iron removal with M representing maghemite and G, goethite. Maghemite

Concentrates G/M mass ratio (g/g)

dosage (g/L)

γ (%)

0

0

1

16.32

2.78

1.78

47.86

2

28.50

5.13

1.57

3

40.14

7.51

4

49.92

5

Mass g/L

Fe (%)

Ca (%)

Tailings Mass Fe (g/L) (%)

Ca (%)

16.16

14.96 12.93

3.12

14.25

12.07

45.52

3.44

12.87

10.31 14.85

1.50

44.32

2.75

11.2

8.64

16.76

9.84

1.46

43.92

2.91

9.87

5.79

18.23

61.51

12.16

1.43

42.66

2.88

7.61

4.45

22.75

6

64.02

13.31

1.22

42.85

2.31

7.48

4.01

23.21

7

64.43

14.24

1.03

43.51

2.45

7.86

3.78

23.54

14.1

Fig. 4 shows an increase in Fe recovery with increasing maghemite dosage up to 5 g/L, where iron recovery leveled at 85%. In contrast, calcium reporting to the concentrates increased slightly, up to 17% at 5 g/L maghemite addition level. It should be noted that only the Fe content of goethite precipitates is considered when calculating Fe recovery because almost all the maghemite particles added would report to the concentrate at 2000 Gs magnetic field strength. Considering the efficiency and cost of the maghemite particles, 5 g/L was considered to be the optimal dosage. At this dosage, the concentrate at 85% iron recovery contains 42.66 wt.% Fe and 2.88 wt.% Ca while the tailing contains 4.45 wt.% Fe and 22.75 wt.% Ca.



100

Fe recovery Ca loss

80

80

60

60

40

40

20

20

0

1

2

3

4

5

6

7

Ca loss to concentrate (%)

100

Fe recovery in concentrate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

Dosage of maghemite fine particles (g/L) Fig. 4. Recovery of Fe and loss of Ca in the concentration of magnetic separation as a function of maghemite dosage. The effect of magnetic field strength on the magnetic separation was also investigated. As shown in Fig. 5, there is a distinct rise in the concentrate yield (γ) with increasing the magnetic field strength from 500 Gs to 1500 Gs, which indicates that the magnetic field strength is too low to separate all the magnetic particles from the suspension. Further increasing the magnetic field strength to 2500 Gs showed little increase in the concentrate yield. The loss of Ca increases continually with increasing field strength. There is a slight dip and an obvious rise in the Ca content of the concentrate with increasing maghemite dosage and magnetic field strength, respectively. This opposite trend indicates that calcium sulfate does not precipitate on maghemite surface but would be entrained in the process of magnetic separation. Therefore 1500 Gs magnetic field strength is a reasonable choice. Accordingly, under this condition, the recovery of Fe and loss rate of Ca are 93.2% and 8.1% respectively in the concentrate.


100

100

80

80 Fe recovery Ca loss

60

60

40

40

20

20

0

500

1000

1500

Ca loss to concentrate (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe recovery in concentrate (%)

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0 2500

2000

Magnetic field strength (Gs) Fig. 5. Recovery of Fe and loss of Ca in magnetic separation with different magnetic field strength. Sulfur and arsenic removal of goethite precipitates The content of all elements of the goethite concentrate collected at 1500 Gs magnetic separation with 5 g/L maghemite addition is shown in Table 3. There are 3.39% As and 6.04% S in goethite concentrate that need to be removed to use this concentrate. The results of chemical phase analysis show that all the As are in the form of iron arsenate (FeAsO4 2H2O) and all of the S are in the form of sulfates, include CaSO4 2H2O, Fe3(HO)5SO4 2H2O, Fe16O16(OH)12(SO4)2 and Zn4(HO) 6SO4.29-31 Table 3. Composition (wt.%) of goethite precipitates (concentrate) and calcium sulfate precipitates (tailings) obtained by XRF analysis. Elements

Fe

Ca

Zn

As

S

O

Others

Concentrate

43.51

2.13

0.26

3.39

5.04

42.94

2.73

Tailings

4.71

22.87

0.02

0.05

18.32

53.92

0.11

Roasting at high temperature is an effective method to remove As and S as gases As2O3 and SO2 by thermal decomposition.32 However, through thermodynamic calculation, the temperatures of the thermal decomposition of iron arsenate and calcium sulfate shown in



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reactions (5) and (6) are around 1600 °C and 1249 °C, respectively. It is not economical to achieve decomposition at such high temperatures for industrial applications of large volume of concentrate treatment. (5) (6) However, the decomposition temperature could be greatly reduced by adding a certain amount of coke powder in the roasting with a limited oxygen atmosphere. In the reduction roasting, coke powder provides little reducing effect because of solid-state reaction of limited contact area and mass transfer.

produced by coke reacting with limited oxygen or carbon dioxide is a more

effective reductant. The reactions in the relevant reduction roasting are shown in reactions (7) to (12). (7) (8) (9) (10) (11) (12) (13) Although As2O3 gas produced in reaction (7) could release arsenic from goethite concentrate, As2O3 gas may transform to elementary arsenic, as shown in reaction (8), which returns to the concentrate. The Gibbs free energy change (

) of the two reactions is shown as following: (14)

where

and

in reaction (7);

these two reactions changes with arsenic removal,

,

,

and

of

and T of the system. In order to improve

of reaction (7) should be decreased while

increased. In addition,

in reaction (8). Overall

of reaction (8) should be

gas was removed from the roaster by 0.01 12 ACS Paragon Plus Environment

nitrogen and

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subsequently collected by exhaust gas collection bottle, further decreasing the content of in the system. For such a complex system, the optimum dosage of coke powder and temperature of the reduction roasting need to be determined by lab tests. As shown in reactions (9), (11) and (12), sulfur in the goethite concentrate can be removed by thermal decomposing of corresponding sulfates species into SO2 gas and oxides. However, CaS produced in this system could have a detrimental effect on sulfur removal as shown in reaction (10).

of the four reactions is shown as follows: (15)

where

and for reaction (11);

for reaction (9); and

and

for reaction (10);

and

for reaction (12). To avoid reaction (10), the

dosage of coke powder and temperature are very critical. The effect of coke powder dosage on S and As removal from goethite concentrate in reduction roasting was studied. The results are shown in Fig. 6. The removal efficiency of S and As increases sharply initially and levels off gradually with increasing the mass ratio of coke powder to goethite concentrates. When

is less than

(the optimum value of

to remove

S and As), increasing the mass ratio of coke powder is beneficial to the thermal decomposition of iron arsenate and sulfates. However, when

is greater than

, reactions (8) and (10) are

promoted by increasing coke powder addition, which inhibits the removal of S and As from the concentrate.


Page 14 of 21

100

3.0

90

2.5 2.0

80 Removal efficiencyof S Removal efficiency of As Grade of S Grade of As

70

1.5 1.0

60 0.5

Grade of As and S (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Removal efficiency of As and S (%)


50 1

2

3

4

5

6

0.0

Coke powder addition (wt.%) Fig. 6. S and As removal from goethite concentrate in reduction roasting at 1100 °C for 30 mins with different dosage of coke powder. Fig. 7 shows the results of S and As removal at roast temperature from 900 °C to 1150 °C. The removal efficiency of As is seen to increase with increasing roasting temperature until 1100 o

C at which all the S is removed from the concentrate. The results indicate that high temperature

facilitate S removal reactions (9), (11) and (12). Even CaS would be formed in reaction (8) at higher temperature, it will further react with gypsum to generate SO2 as reaction (13). The removal efficiency of As also improves with increasing roast temperature up to 1050 °C, which can be attributed to enhanced arsenic removal reaction (7). At roast temperature above 1100 oC, further increase in roast temperature caused a slight decrease in arsenic removal, indicating facilitating reaction (8) against arsenic removal reactions when temperature is more than 1100 °C.


100

2.5

90

2.0

80

70

Removal efficiency of S 1.5 Removal efficiency of As Grade of S 1.0 Grade of As

60

0.5

50

900

950

1000

1050

1100

1150

Grade of As and S (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As and S removal efficiency (%)

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0.0

Roasting temperature (oC) Fig. 7. S and As removal from goethite concentrate in reduction roasting at coke powder to concentrate mass ratio of 4% and different roasting temperature for 30 min. Under the optimum condition (coke powder to concentrate mass ratio of 4% and roasting temperature of 1100 °C), the removal efficiency of S and As can reach 99.3% and 99.5% at the S and As grades in goethite concentrate around 0.035 wt.% and 0.017 wt.%, respectively. The results demonstrate that reduction roasting is a very effective approach to remove S and As from goethite concentrate produced by magnetic separation from the product of goethite process of zinc oxide dust leachate. After the reduction roasting, the result of all elemental analysis of the goethite concentrate is shown in Table 4. The Fe grade of the concentrate up to 68.52% meets the requirement of ironmaking raw material. The remaining 3.35% Ca in the form of CaO could be used as a source of lime needed in the subsequent ironmaking process to remove S, P and slagging.33, 34 However, the concentrate still contains 0.40% Zn which will accumulate in blast furnace and cause challenges in the ironmaking process.35 Technologies to deal with Zn in the ironmaking process 35

need to be considered or the concentrate be used by mixing with other non-zinc iron ores.

Table 4. Composition of goethite concentrate after roasting determined by X-ray fluorescence. Elements

Fe

Ca

Zn

As

S

O

Wt. %

68.52

3.35

0.40

0.017

0.035

27.72



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Application of calcium sulfate residue There are many reports about the application of gypsum by-product.36,

37

The tailings of

magnetic separation is dried to produce gypsum solids. As shown in Table 3, there contains only a small quantity of goethite in the gypsum solids which can be directly used in cement and building materials production. Conclusions The residues of the goethite process in the zinc oxide dust hydrometallurgy contain toxic and heavy metallic elements. Impounding these residues in tailings ponds causes serious environment concerns. In this study, a novel and robust method based on the addition of maghemite fine particles as the crystal nucleus of goethite precipitates in the goethite process was proposed and tested. The goethite precipitates formed on the surface of maghemite particles became magnetic that can be separated into goethite concentrate and calcium sulfate tailings by magnetic separation. S and As contained in the goethite concentrate were removed effectively by reduction roasting with 4% coke powder at 1100 °C. The resulting goethite concentrate containing 68.52% Fe, 0.017% As and 0.035% S can be used as ironmaking raw material. The calcium sulfate (gypsum) tailings generated with a small quantity of goethite, on the other hand, can be used in cement and building materials production. The technology developed in this study provides a feasible way to convert a hydrometallurgy waste of zinc hydrometallurgy to value-added raw materials for the ironmaking and production of building materials. Acknowledgement This work was supported by Sublimation Scholar’s Distinguished Professor of Central South University, the Innovation Driven Plan of Central South University (No: 2015CX005), and the National 111 Project (No: B14034), Collaborative Innovation Center for Clean and Efficient utilization of Strategic Metal Mineral Resources.


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For Table of Contents Use Only Magnetic seed crystal, magnetic separation and reduction roasting are applied to treat and recycle the iron oxide residues in zinc hydrometallurgy for the sustainable development of resources and environment.


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338x190mm (300 x 300 DPI)

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Magnetic Separation and Recycling of Goethite and Calcium Sulfate in

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